The invention relates to an auditive prosthesis, comprising a cochlear implantable electrode-set applied on a carrier, which electrode-set is provided for applying electrical currents for stimulating auditory nerves in the modiolus, wherein the electrode-set comprises a first series of electrode-poles arranged beside each other and at a distance of each other along a first longitudinal side of the carrier, each of the electrode-poles of said first set are individually connected with a signal-generator, which electrode-set comprises a second electrode-pole connected with the signal-generator, which second electrode-pole is arranged along a second longitudinal side of the carrier.
Such an auditive prosthesis is known from the German patent application 28 23 798. The known prosthesis is implanted in the cochlear of a patient having auditory problems in order to improve his auditory perception. By means of the electrodes, currents are applied, under control of received sound-signals, which currents stimulate the auditory nerves in the modiolus, in order to induce action-potentials in those nerves, which action-potentials will thereafter be transported to the brains for interpretation of the audio-signals. With the known prosthesis the electrode-set is formed of the first and second point-shaped electrode-poles, wherein each electrode-pole is each time connected via a wire with the signal-generator. The second electrode-poles form the earth.
A problem of the known auditive prosthesis is that each electrode-pole has each time to be connected with an individual wire with the signal-generator. In order to obtain an increased spacious resolution of the stimulation-pattern, it is necessary to have a plurality of first poles. In view of the limited space available in the scala tympani, the requirement of each time a wire for each pole leads to a limitation in the number of poles and thus to a limited spacious resolution. By using moreover for the first as well as for the second electrode-poles each time point-shaped electrodes, the geometry of the field-lines to be applied is limited.
It is an object of the invention to realise an auditive prosthesis wherein stimulation with a higher spacious resolution is possible.
An auditive prosthesis according to the invention is therefore characterised in that, the second electrode-pole extends over substantially the same distance as the one over which the first electrode-poles extend and which signal-generator is provided for generating signals which generate between electrode-poles to be selected, electrical field-lines with radial and/or longitudinal components. Due to this the second electrode-pole forms a collective pole and thus only a single wire is necessary for connection with the signal-generator. When the first pole comprises n electrodes only n+1 wires are required, due to which a plurality of first electrode-poles is possible, without having the limited space available in the scala tympani forming too quickly a problem. The combination of the first electrode-poles and the second collective pole offers moreover the possibility to generate by means of the signal-generator signals enabling to generate radial as well as longitudinal field-lines. Using those two kinds of field-lines enables to increase the spacious resolution.
It is favourable that said second pole is longitudinally configured. The configuration of the second electrode-pole thus allies to the one of the carrier.
A first preferred embodiment of an auditive prosthesis according to the invention is characterised in that said signal-generator is provided for generating bi-phase-stimulation-pulses of which a first and second phase have substantially a same time-duration but are opposite to each other. The use of the collective second pole with bi-phase-stimulation enables a better control of the field-lines.
A second preferred embodiment of an auditive prosthesis according to the invention is characterised in that said electrode-set comprises at least one selectable reference-electrode and wherein a feedback-element is applied between the selected reference-electrode and the generator, which feedback-element is provided for reducing to zero the net current measured over a current pulse-period. This enables to avoid direct currents over the electrode-contacts which could be unfavourable for the modiolus.